An obstetrician we know recently contacted us. She was 30 weeks’ pregnant with her first baby, and concerned about her running routine this pregnancy. Before she was pregnant, she ran regularly, and had continued until this point. But, despite browsing the available literature, she felt uncertain as to whether it was safe to keep it up, and was finding it increasingly difficult to justify her continued urge to run in response to family and friends’ comments.

This is a typical scenario, which is frequently brought to our attention. Coupled with adverse comments in the popular press about selfish behaviours of women wishing to run, it provided the impetus for us to evaluate the situation further. What should women do if they want to participate in recreational, and even competitive, running during pregnancy?

IN SEARCH OF GUIDELINES

The Royal College of Obstetricians and Gynaecologists’ guidelines recommend 150 minutes of moderate intensity exercise each week, in the absence of obstetric complications,¹ based on evidence of no harm to mother or fetus.^2,3 But there is limited specific guidance related to running – a more vigorous, weight-bearing activity, which may have unique effects on the developing fetus and female body, antenatally and postpartum. The evidence base is small, with existing studies employing varied methodologies and definitions, making it difficult for health professionals to draw robust conclusions.

Currently, it seems that uncertainty prevails amongst pregnant women who either stop or drastically cut back on antenatal running routines because of a lack of knowledge surrounding the potential risks, as well as clinicians – in a recent study reporting reasons women stopped running during pregnancy, almost a third did so based on advice from their doctor.⁴

EXERCISE AND BIRTHWEIGHT

Moderate intensity aerobic exercise throughout pregnancy is known to result in a lower caesarean section rate, reduced incidence of gestational diabetes and hypertensive disorders, decreased maternal weight gain and improvements in antenatal and postnatal depression, and has not been found to adversely affect birthweight.^5–7

Studies investigating more intense exercise have reported mixed effects on birthweight, which have potentially important clinical implications, as birthweight is the single most important predictor of neonatal morbidity and mortality.⁸ Decreases in utero-placental blood flow occur during vigorous and high intensity exercise (defined as being at least 70% of maximum heart rate (MHR), or an activity where normal conversation can’t be maintained⁹), as blood is redirected towards skeletal muscle. By the third trimester, uterine blood flow has increased from 50ml to 500ml/minute and there are theoretical concerns that regular, strenuous exercise, with associated redistribution of blood flow away from the developing fetus could result in impaired growth or other fetal consequences.

However, a recent systematic review and meta-analysis including 15 studies reported no significant difference in birthweight in infants of mothers who engaged in vigorous physical activity compared with those who didn’t. In fact, a Cochrane review of 14 trials (1014 women) reported a reduced risk of large for gestational age (>4000g) and small for gestational age (<2500g) babies, both known to be associated with morbidity.¹⁰ Lower birthweight, on average 200g, was reported for women who exercised into their third trimester, but this reflected reduced fat mass and increased lean body mass,¹¹ which is likely to be associated with better long term health outcomes.

EXERCISE AND PRETERM BIRTH

Running is a weight-bearing exercise, generating considerable forces as the foot strikes the ground, which are transmitted through the limbs and pelvic girdle, with potential contrecoup effects within the uterus that could affect cervical integrity and potentially increase the risk of preterm birth (delivery <37 weeks).

However, no studies have reported an increase in preterm birth, including our own investigation into the running habits of 1293 women during pregnancy, 15% of whom ran into their third trimester. This study was also reassuring, given that recreational and other running was not associated with any detrimental effects on birthweight.¹²

EXERCISE AND ASSISTED BIRTH

General muscle hypertrophy and increased muscle tone attributed to regular exercise could theoretically predispose recreational and elite athletes to a prolonged second stage of labour and increased assisted delivery rates, related to soft tissue resistance.

We found that the assisted vaginal delivery rate was significantly increased, albeit marginally, in women who continued to run during pregnancy (27%) compared with those who stopped (25%).¹² This may be attributable to hypertrophied and toned pelvic floor muscles blocking the downward passage of the fetus during pushing.

EXERCISING AFTER DELIVERY

The physiological effects of pregnancy are generally thought to persist for 4–6 weeks postpartum and, aside from pelvic floor exercises, women are not typically encouraged to return to strenuous exercise regimes, including running.

However, this is an arbitrary time point, and many women return to exercise earlier, which has been associated with reduced risk of postpartum depression.⁴ In terms of physical effects, theoretically, pelvic floor muscles may be overloaded if they are not fully recovered before a return to strenuous exercise, and early return to heavy physical work after childbirth increased risk of urinary incontinence and pelvic organ prolapse in small populations of women when studied in India¹³ and Nepal¹⁴ respectively. But a consensus statement by Bø et al. on exercise in the postpartum period identified no clear evidence of an increased risk of pelvic floor injury.¹³

IN SUMMARY

Continuing to exercise vigorously during pregnancy, including running, does not appear to be harmful to either mother or baby, and this is what we told our colleague. The evidence base currently suggests that obstetricians can confidently recommend that pregnant women continue to run, and engage in other forms of intense exercise, as long as there isn’t a risk of direct trauma to the uterus. It also remains likely that exercise in pregnancy has beneficial effects that outweigh any theoretical risks.

Katy Kuhrt, Academic Clinical Fellow, St Thomas’ Hospital and King’s College, London

Andrew Shennan, Professor, Obstetrics and Gynaecology, St Thomas’ Hospital and King’s College, London

REFERENCES

1. UK Chief Medical Officers 2019 Physical Activity Guidelines www.gov.uk/government/collections/physical-activity-guidelines.
2. Melzer K et al. 2010 Sports Medicine 40 493–507.
3. Prather H et al. 2012 PM&R 4 279–283.
4. Tenford A et al. 2015 Sports Health 7 172–176.
5. Di Mascio D et al. 2016 American Journal of Obstetrics & Gynecology 215 561–571.
6. Daley AJ et al. 2015 BJOG 122 57–62.
7. Pyatos-León R et al. 2017 Birth 44 200–208.
8. Bell R 2002 Journal of Science & Medicine in Sport 5 e8551.
9. Norton K et al. 2010 Journal of Science & Medicine in Sport 13 496–502.
10. Kramer MS & McDonald SW 2006 Cochrane Database of Systematic Reviews doi:10.1002/14651858.CD000180.pub2.
11. Siebel AL et al. 2012 Clinical & Experimental Pharmacology & Physiology 39 944–957.
12. Kuhrt K et al. 2018 BMJ Open Sport & Exercise Medicine 4 e000296.
13. Prabhu SA & Schanbhag SS 2013 Journal of Research in Health Sciences 13 125–130.
14. Lien YS et al. 2012 International Journal of Gynaecology & Obstetrics 119 185–188.